You should be bovvered .... IMO. In a previous life I have filled out the claim forms to "sue" a solicitor and he paid up immediately.
OK a plan:-
Go round in the evening, with your wife (a child is better!

) and ask what was wrong and what can you do to put it right; tell the lady that she has an obligation to tell you what was wrong and let you put it right.
Let's assume she's not honest and pulling a fast one.
You now hand her a letter and say "I have done the job well, I beleive you owe me the £XXX and I will now pursue this matter further." Say nothing more but repeat this exactly whatever she threatens you with.
Walk away, stand at the bottom of her drive looking back at the house and taking notes and talking into a mobile phone.
(The letter should say what you did, the amount, also you have not been allowed to resolve any issues and that if not paid within seven days you will "escalate" the matter, and that "for the avoidance of doubt" any phone calls from them may be recorded.)
See if anything happens. Take notes.
Always be polite but firm.
They may ignore you, he may send a letter denying etc. if he phones with a "not paying push off attitude" just say "I will now escalate this matter, good day!" and put the phone down.
At worst it will make them sweat!